For making a noun plural normally "s" is added at the end of the word (exceptions exist). The plural forms of definite article are "os" (masculine) and "as" (feminine). Here again the article must agree in gender and number with its noun. Let's see some examples:

os gatos	(The cats, masculine plural)
os homens	(The men, masculine plural)
as canetas	(The pens, feminine plural)
as trovoadas	(The thunders, feminine plural)